Stabilized braided biaxial structure and method of manufacture of the same

ABSTRACT

A flat broad good is manufactured from a portion of an intermediate braided structure laid into flat broad good form. The portion forming the flat broad good is a stabilized braided biaxial structure.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to and the benefit of U.S. Provisional Patent Application No. 62/023,445 filed Jul. 11, 2014, the contents of which are hereby incorporated by reference in their entirety.

BACKGROUND

This relates generally to braided textile structures. Braided textile structures are often embedded in a resin matrix to form a final composite article and thereby act as reinforcement to the final composite article and impart the final composite article with certain predetermined and possibly varying tensile strengths in different directions.

Braided biaxial structures are typically comprised of two groups of tows of material laid into the structure in an interleaving pattern wherein one group of tows is aligned along an oblique direction relative to the longitudinal direction of the structure and the other group of tows is aligned along an opposing oblique direction. The opposing oblique directions and the longitudinal direction are principal directions in the textile architecture.

Braided textile structures are commonly manufactured in generally cylindrical forms, such as sleeves, or flat forms, such as tapes. In sleeve form the longitudinal direction lies along the sleeve axis and the opposing oblique directions form opposing helices wrapping around the longitudinal direction. In tape form, the tows aligned along one oblique direction wrap around the edge of the tape and the tow direction changes to the opposing oblique direction until the wrapping around the opposing edge of the tape.

A characteristic of a braided biaxial structure is that it generally elongates in the longitudinal direction and contracts in the transverse direction when subjected to longitudinal forces or the longitudinal components of overall forces generally. Conversely, the structure generally contracts in the longitudinal direction and expands in the transverse direction when subjected to transverse forces or the transverse components of overall forces generally. The change in macroscopic geometry is generally due to the interconnectedness of the interleaved groups of tows. The corresponding geometric change within the structure may be characterized by variations in the angles between tows lying in oblique and longitudinal directions.

In one example, some braided biaxial sleeves can envelop and conform to bodies of varying cross section. The conformability and continuity of these sleeve structures around a body of varying cross section provides for placement of tow materials along paths may correspond to varying principal directions of the enveloped body in a predetermined manner.

In another example, some braided biaxial tapes may generally have less utility in this regard, as compared to some braided biaxial sleeves, due to the structural discontinuity represented by the edges of the tape.

While advantageous in some applications, this conformability characteristic can be disadvantageous in other applications, for example, when it is desired to maintain the orientation of the oblique directions within a narrow tolerance range within regions of the final composite article or across the entire article as a whole. An example of one such application is large aerospace structures including ±45 degree braided biaxial structure reinforcements in a resin matrix.

In some applications of the example above, it is desired that the behavior of the final composite structure be closely approximated by the behavior of well-defined test structures. These test structures generally include coupons with predetermined dimensions and structure subjected to load, environmental and life tests to develop a database of material properties. The material properties of these structures are dependent on many factors and composite structure designers must take care in ensuring that final composite structures are similar enough to the test coupon structures that the materials properties in the database can be used with some level of confidence. In general, triaxial braided structures, wherein the structure is comprised of two tows aligned along opposing oblique directions and a third tow aligned along the longitudinal direction, may be preferred in certain applications as the longitudinal tows tend to lock the structure, restricting the amount of deformation, and ensure that the structure deployed in the final composite article will closely approximate the structure of the test coupons.

In aerospace parts, which are highly scrutinized for weight limitations, cost limits and other constraints, a triaxial structure may not be optimum due to the additional material laid in the longitudinal direction, particularly in applications where the longitudinal material provides little to no additional desired load carrying capability. A biaxial structure with low deformability characteristics may be preferred to a triaxial structure for some of these types of parts. For example, a biaxial structure with limited, predictable deformability may be preferable for these parts when a limited amount of drapability is required.

One example of a braided biaxial structure for some applications includes tows of carbon or other high strength fiber aligned along the opposing oblique directions and thermoplastic and lower strength fibers, such as glass fibers, aligned along the longitudinal direction. The glass material provides enough tensile strength to facilitate manufacture of the braided structure without undue distortion. The thermoplastic material may be melted, or semi-melted, by application of heat and allowed to cool thereby limiting the deformability of the braided structure. Development of materials properties data relevant to said structures is costly and time-consuming and few generally available materials properties data are available. Therefore, these braided structures, though generally not expected to deviate in properties from a biaxial structure of similar material and geometric architecture, may not be suitable for some applications. Therefore, there is a need for a true biaxial structure that is stabilized such that distortion of the braid is limited to a predetermined range allowing for use of available materials properties data without incurring additional testing.

SUMMARY

The present subject matter relates to stabilized braided biaxial structures manufactured as flat broad goods. The present subject matter also relates to a method of manufacture of these structures.

A braided biaxial structure, generally supplied as a broad good, includes interleaved tows of material in opposing oblique directions and further includes a matrix soluble thermoplastic epoxy coating on at least one side of the structure.

A method of manufacture includes a step where an intermediate braided structure is manufactured in a conventional sleeve form. The structure of the intermediate braided structure may be generally biaxial and include at least one triaxial region. The triaxial region may be formed from thermoplastic tow materials laid in the longitudinal direction. The triaxial region may further include materials laid in the longitudinal direction that are generally of lower strength, and possibly also lower cost, as compared to the tow materials in the oblique directions. It is expected that the materials will carry tensile forces induced by the manufacturing apparatus within the braided structure and prevent distortion of the biaxial regions of the structure.

A method of manufacture may include a step where heat is applied to a triaxial region of a structure to affect melting of longitudinal thermoplastic fibers thereby limiting distortion of a biaxial region of the structure during subsequent manufacturing steps.

A method of manufacture may include a step where a matrix soluble thermoplastic epoxy coating is applied to at least one side of a broad good and heated to solidify the coating. The duration and intensity of heat may be controlled so as to prevent cure of the coating and retain the coating as matrix soluble for when a braided biaxial structure is deployed as reinforcement in a composite article.

A method of manufacture may include a step to trim away a triaxial region or regions of a structure resulting in a stabilized braided biaxial structure that may withstand tensile forces applied during deployment as reinforcement in a composite article without distortion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a stabilized braided biaxial structure.

FIG. 2 is a transverse cross sectional view of a portion of the stabilized braided biaxial structure of FIG. 1.

FIG. 3 is a front view showing an intermediate braided structure in the manufacture of the stabilized braided biaxial structure of FIG. 1.

FIG. 4 is a plan view of the intermediate braided structure of FIG. 3 in a slit state.

FIG. 5 is a flowchart of a method of manufacture of the stabilized braided biaxial structure of FIG. 1.

FIG. 6 is a view similar to FIG. 3 showing an intermediate braided structure according to another embodiment.

FIG. 7 is a flowchart of a method of manufacture of a stabilized braided biaxial structure incorporating the intermediate braided structure of FIG. 6.

FIG. 8 is a schematic illustration of the applying and curing of thermoplastic epoxy coating of the method of FIG. 5.

FIG. 9 is a schematic illustration of the curing of thermoplastic epoxy coating according to another embodiment.

FIG. 10 is a plan view of the intermediate braided structure of FIG. 6 in a slit state.

FIG. 11 is a schematic view of a multilayer lamination process according to a further embodiment.

FIG. 12 is a schematic view of an application of epoxy according to yet another embodiment.

DETAILED DESCRIPTION

As used in the disclosure, a “braided structure” is a structure including a plurality of strands of material, commonly called tows, such that each tow is intertwined with at least one other tow in a repeating pattern. Two-dimensional braided materials are those wherein the repeating pattern is largely characterized by two or more principal weave directions all in a common plane, typically the longitudinal direction of the braided structure, commonly called the axial direction, and one or more oblique directions, commonly called bias directions, each at a predetermined angle to the longitudinal direction. Three-dimensional braided structures are those wherein additional principal directions, generally mutually perpendicular to the longitudinal and oblique directions, are required to completely define the structure and the patterns thereof. For simplicity of description, these additional directions are generically referred to as radial directions, whether the structure is generally tubular in form, laid out as a flattened tubular form or in a fabric, or generally planar, form. Herein, reference to braided structure generally implies two-dimensional forms but does not exclude three-dimensional forms.

Two-dimensional braided structures may be manufactured as generally cylindrical materials, commonly called sleeves, with the axial direction corresponding to the longitudinal axis of the cylinder and the bias directions oblique to the longitudinal axis. Braided structures manufactured in cylindrical form may then be laid-flat to form a two-dimensional fabric comprised of two layers joined along the longitudinal edges. The edges may be removed to form two separate and distinct layers. One edge may be removed and the cylindrical structure laid-flat to form a singly-slit single layer structure. Two edges may be removed to form a double-slit two-layer structure. Two-dimensional braided structures may further be manufactured in a single layer flat form, commonly called a tape.

In the art, the terms “strand”, “tow”, “yarn”, “yarn bundle”, “fiber” and “fiber bundle” are generally meant to describe what is laid into or intertwined in each of the principal directions of a braided structure. In this disclosure, the term “tow” will generally be used to describe what is laid into or intertwined in each of the principal directions of a braided structure. A tow is an amalgamation of all material that runs together in a principal direction. A tow can comprise monofilaments, multiple filaments or be comprised of staple, or spun, material. Tow material can have a variety of cross-sectional shapes, including but not limited to, generally circular, ellipsoidal, triangular, and flat tape shapes. Tow material may be subject to intermediate or pre-processing prior to braiding operations. Examples of intermediate or pre-processing may include, but are not limited to, twisting, braiding small numbers of filaments into braided tow materials, pre-impregnation with resins and specialty coating to facilitate braiding and/or subsequent processing. A tow can comprise any combination of these materials and material forms. Any one tow may comprise one or more filament or staple materials. A tow may be comprised of carbon materials, basalt, glass materials, thermoplastic polymeric materials, thermoset polymeric materials, a combination of carbon and polymeric materials or a combination of polymeric and glass materials, or some combination thereof, and other suitable materials. Tows that lay in one of the bias directions of the fabric are commonly called bias tows. Tows that lay in the axial direction of the fabric are commonly called axial tows.

Biaxial braid is generally formed from bias tows. Triaxial braid generally includes bias and axial tows. Hybrid braided structure may be formed of contiguous materials including one or more adjacent regions of biaxial and triaxial braid.

Referring now to the drawings, there is illustrated in FIG. 1 a stabilized braided biaxial structure 11 with principal longitudinal direction 12 and opposing oblique directions 13 and 14. Illustrated in part, a first group of tows 15 are aligned along the first oblique direction 13 and a second group of tows 16 are aligned along the second oblique direction 14.

As best shown in FIG. 2, the stabilized braided biaxial structure 11 includes a matrix soluble thermoplastic epoxy coating 21 affixed to one surface of the braided biaxial structure 11.

There is shown in FIG. 3 an intermediate braided structure 31 formed in a process for manufacturing the braided biaxial structure 11. As illustrated, the intermediate braided structure 31 is a hybrid structure in sleeve form including a biaxial region 32 and a triaxial region 33 with thermoplastic tows 34 laid in along the longitudinal direction. Processing tows 35 may also be laid into the longitudinal direction along with the thermoplastic tows 34. The amalgamation of thermoplastic tows 34 and processing tows 35 may herein be referred to commonly as longitudinal tows 34, 35. In the process of manufacture, a method for which is further described below, the intermediate braided structure 31 may be slit along a cut line, indicated at 36, and then laid flat to form an intermediate broad good 41, as shown in FIG. 4, with central biaxial region 42 formed from the biaxial region 32 and edge triaxial regions 43 formed from the triaxial region 33.

There is illustrated in FIG. 5 a method 50 for the manufacture of a stabilized braided biaxial structure. In a first step 51, an intermediate braided structure of hybrid form with biaxial and triaxial regions is manufactured. The triaxial region includes axial or longitudinal tows comprised of thermoplastic fibers and may also include processing tows. It is expected that the processing tows will reduce distortion in the biaxial region due to tensile forces imposed on the structure during manufacture.

In a second step 52, heat is applied to the axial tows in the triaxial region of the structure to melt the thermoplastic tow materials thereby affixing in place the portions of the bias tows passing through the triaxial region.

In a third step 53, the intermediate braided structure is slit along a cut line in the triaxial region.

In a fourth step 54, the intermediate braided structure is laid flat into a broad good form.

In a fifth step 55, a matrix soluble thermoplastic epoxy coating is applied to at least one surface of the broad good form. The matrix soluble thermoplastic epoxy coating may be applied in powder or liquid form.

In a sixth step 56, heat is applied to the matrix soluble thermoplastic epoxy coating to solidify the coating with or without curing the resin. The solidified matrix soluble thermoplastic epoxy coating serves to lock the orientation of the bias tows in the biaxial region of the braided structure.

In a seventh step 57, the triaxial regions of the structure are trimmed off thereby leaving a stabilized braided biaxial structure.

In an alternative method of manufacture may also include an optional step, not shown, between laying the slit sleeve flat, the fourth step 54, and applying the matrix soluble thermoplastic epoxy coating, the fifth step 55, where the laid flat broad good is wound onto a reel and the reel is then moved to another set of apparatuses for performance of subsequent steps.

An alternative intermediate braided structure 61 is shown in FIG. 6. The alternative intermediate braided structure 61 is a hybrid structure in sleeve form and includes at least two biaxial regions 62 and at least two triaxial regions 63. In a method of manufacture for the alternative intermediate braided structure 61, the alternative intermediate braided structure 61 may be slit along cut lines 64 and the resulting sections laid flat to each form an intermediate broad good similar to that shown in FIG. 4.

There is illustrated in FIG. 7 another method 70 for the manufacture of a stabilized braided biaxial structure. The method 70 may utilize the intermediate braided structure 61 of FIG. 6.

The method 70 includes a first step 71 an intermediate braided structure of hybrid form with at least two biaxial regions and at least two triaxial regions is manufactured. Each triaxial region includes axial or lateral tows including thermoplastic fibers and may also include processing tows. It is expected that the processing tows will reduce distortion in adjacent biaxial regions due to tensile forces imposed on the structure during manufacture.

In second step 72, heat is applied to the axial tows in each triaxial region of the structure to melt the thermoplastic tow materials thereby affixing in place the portions of the bias tows passing through each triaxial region.

In third step 73, the intermediate braided structure is slit along a cut line in each triaxial region.

In a fourth step 74, each resultant section of the structure is laid flat

In a fifth step 75, a matrix soluble thermoplastic epoxy coating is applied to at least one surface of each resultant section of the structure. The matrix soluble thermoplastic epoxy coating may be applied in powder or liquid form.

In a sixth step 76, heat is applied to the matrix soluble thermoplastic epoxy coating on each resultant section to solidify the coating with or without curing the resin.

In a seventh step 77, the triaxial regions of each resultant section of the structure are trimmed off thereby leaving each resultant section of the structure as a stabilized braided biaxial structure.

There is illustrated in FIG. 8 one process of applying thermoplastic epoxy coating. In the illustrated process, epoxy is transferred to a braided material 81 by utilization of gravity application. The process includes the braided material 81 passing through an epoxy application chamber 82, wherein uncured epoxy is released from an epoxy depositor 83 and transferred to the braided material 81 via gravity feed, as shown at 84, and thus accumulating on the surface of the braided material 81. The applied uncured epoxy 85 on the braided material 81 travels past a heater 86 which cures the thermoplastic epoxy from an uncured state, as shown at 85, to semi-cured state, as shown at 87, and optionally to a fully-cured state, as shown at 88, thus producing a stabilized biaxial braided structure.

There is illustrated in FIG. 9 a thermoplastic curing process. In the illustrated process, braided material 91 has uncured thermoplastic epoxy 92 applied. The uncured epoxy 92 on the braided material 91 travels past a heater 93 which cures it from an uncured state 92, to semi-cured state 94, and optionally to a fully cured state 95, producing a stabilized biaxial braided structure.

An alternative method of manufacture may include a step where a resultant section is wound onto a reel and the reel is then moved to another set of apparatuses for performance of subsequent steps. Such a step may occur between laying the resultant section flat and applying the matrix soluble thermoplastic epoxy coating to the resultant section.

Another alternative method of manufacture may include manufacturing an intermediate braided tape structure, as opposed to an intermediate braided sleeve structure, with triaxial regions as described in the methods above. The intermediate braided tape structure may include triaxial regions along both edges of the tape and biaxial region between the two edges. A mechanism may be utilized to ensure the orientation of the bias tows after the braided tape structure has fully formed. The mechanism may be crowned rollers, bowed rollers, tension adjustment mechanisms, and or any other suitable mechanisms. Once the intermediate braided tape structure is manufactured, heat may be applied to the triaxial regions to melt the thermoplastic tow materials and lock the edges of the tape. Following, a matrix soluble thermoplastic epoxy coating may be applied and solidified and then the triaxial regions trimmed away.

An alternate embodiment of the method of manufacture utilizing an intermediate tape structure comprises an intermediate tape structure with at least two biaxial regions and three triaxial regions slit into at least two structures. As best shown in FIG. 10, the intermediate tape structure 100 consists of triaxial regions 101 and biaxial regions 102. Within the central triaxial region 101 is a cut line 103 for further processing and separation of sections of the tape structure 100 (shown as consisting of a biaxial regions paralleled with a triaxial regions).

Other alternate embodiments may include applying and heating thermoplastic film over regions of biaxial braid to fix or lock the orientation of the bias tows in place of thermoplastic tow materials laid into the braid. Thermoplastic films may also remove the need for processing tow materials laid into the biaxial regions where cuts will be made.

Further alternate embodiments may include multi-layer lamination of braid materials as shown in FIG. 11. An initial braided material layer 111, for example un-stabilized biaxial braided material layer, receives uncured thermoplastic epoxy 112 a. The initial braided material layer 111 with the thermoplastic epoxy 112 a travels past a primary heater 113 a, producing a semi-cured epoxy 114 a. While the epoxy is in a semi-cured state as semi-cured epoxy 114 a, an additional layer of braided material 116, for example another un-stabilized biaxial braided material layer, is applied to the initial layer 111. The heater 113 a further cures the epoxy to a fully-cured state epoxy 115 a, generally bonding the two layers 111 and 116 together. An additional layer of thermoplastic epoxy 112 b may be added to the additional braid layer 116. The additional braided material layer 116 may then receive uncured thermoplastic epoxy 112 b. The additional braided material layer 116 with the thermoplastic epoxy 112 b may then travel past a second heater 113 b, producing a semi-cured epoxy 114 b. The second heater 113 b may further cure the epoxy with to a fully-cured state epoxy 115 b, generally bonding the two layers 111 and 116 together. Further optional layers of braid or epoxy may be added to further stabilize or reinforce the resultant product. While the above heating was described with two heaters, it must be understood that the heating may be performed by one heater or any number of multiple heaters.

Another alternate embodiment includes application of an epoxy through fluid assistance. As shown in FIG. 12, as braided material 121 passes into an epoxy application chamber 122, uncured thermoplastic epoxy 124 is released by an epoxy depositor 123. The uncured thermoplastic epoxy 124 may be generally dispersed via gravity. During descent of the uncured thermoplastic epoxy 124, fluid nozzles 125, which are preferably offset, deliver a pressurized fluid into the chamber 122. The fluid may be or include an inert gas or atmospheric fluid. The offset angle of the nozzles and velocity of the fluid causes the gravity-dispersed epoxy 124 to become suspended in the chamber 122. The suspended epoxy 126 may be transferred to the braided material 121, by the theory that the braided material 121 is a semi-permeable structure allowing fluid from the pressurized chamber 122 to pass through yet capture suspended epoxy 126 on the braided material 121, creating an uncured epoxy layer 127. This method allows for application of atmospheric sensitive epoxies and even coverage regardless of process orientation.

Additional other alternate embodiments may include using a matrix soluble thermoplastic epoxy in place of thermoplastic tow materials or thermoplastic films, with or without processing tow materials. Further alternate embodiments replacing thermoplastic tows or film with the matrix soluble thermoplastic epoxy may include steps to apply the coating across the entire braided structure once the braid is formed and then to apply heat only where cuts will be made, laying the structure or sections of the structure flat and then to apply heat across the entire surface or surfaces to lock the structure or structures in place. These alternate embodiments may exclude a step of trimming away regions of material employed to hold the structure in place until the matrix soluble thermoplastic epoxy coating is solidified, thereby increasing production yield.

While principles and modes of operation have been explained and illustrated with regard to particular embodiments, it must be understood, however, that this may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope. 

What is claimed is:
 1. A method of manufacturing a flat broad good comprising providing an intermediate braided structure; and laying at least a portion of the structure into flat broad good form; where the portion forming the flat broad good is a stabilized braided biaxial structure.
 2. The method of claim 1 where the intermediate braided structure is a generally biaxially braided sleeve including at least two tow materials in oblique directions and has at least one triaxially braided region.
 3. The method of claim 2 where the triaxial region includes thermoplastic tow materials laid in a longitudinal direction.
 4. The method of claim 3 where the thermoplastic tow materials laid in a longitudinal direction are generally of lower strength as compared to the tow materials in the oblique directions.
 5. The method of claim 4 further comprising: applying heat to the triaxial region of the structure to affect melting of the longitudinal thermoplastic tow materials.
 6. The method of claim 5 further comprising: slitting the intermediate braided structure along a cut line in the triaxial region.
 7. The method of claim 6 further comprising: trimming away at least one section of the triaxial region.
 8. The method of claim 1 where the intermediate braided structure is a generally biaxially braided tape including at least two tow materials in oblique directions and has at least one triaxially braided region.
 9. The method of claim 8 where the triaxial region includes thermoplastic tow materials laid in a longitudinal direction.
 10. The method of claim 9 where the thermoplastic tow materials laid in a longitudinal direction are generally of lower strength as compared to the tow materials in the oblique directions.
 11. The method of claim 10 further comprising: applying heat to the triaxial region of the structure to affect melting of the longitudinal thermoplastic tow materials.
 12. The method of claim 11 further comprising: slitting the intermediate braided structure along a cut line in the triaxial region.
 13. The method of claim 11 further comprising: trimming away at least one section of the triaxial region.
 14. The method of claim 1 further comprising: applying a matrix soluble thermoplastic epoxy coating to at least one side of the broad good; and heating the coating.
 15. The method of claim 14 where the coating is heated to a full cure.
 16. The method of claim 1 where the intermediate braided structure is generally biaxially braided including at least two tow materials in oblique directions and has at least one triaxially braided region including thermoplastic tow materials and processing tow materials laid in a longitudinal direction.
 17. An intermediate braided structure comprising: at least two tow materials biaxially braided in oblique directions; and at least one other tow material laid in a longitudinal direction forming a triaxially braided region.
 18. The intermediate braided structure of claim 17 where the structure is a sleeve.
 19. The intermediate braided structure of claim 17 where the structure is a tape.
 20. A braided biaxial structure, generally supplied as a broad good, comprising: interleaved tows of material in opposing oblique directions; and a matrix soluble thermoplastic epoxy coating on at least one side of the structure. 